An avalanche photodiode is a light detector which typically comprises a .pi.-type conductivity substrate having an n-type layer extending into the substrate a distance from a first major surface of the substrate, and a p-type layer extending a further distance into the substrate from the n-type layer, thereby forming a p-n junction between these two layers. A p.sup.+ -type conductivity contacting layer extends a distance into the substrate from second surface of the substrate to provide a surface layer suitable for electrical contacting. When a reverse bias voltage is applied to the photodiode the depletion region of the diode, i.e., that region having no free electrical charge, reaches through to the .pi.-type substrate region when the peak electric field at the p-n junction is about 5-10% less than that required to cause an avalanche breakdown. A further increase in the applied voltage causes the depletion region to increase rapidly towards the p.sup.+ layer, while the electric field throughout the device increases relatively slowly.
Light entering the device through the p.sup.+ contacting layer is absorbed creating electrical charge. Electrons created in the .pi.-type region are swept to the high field region where multiplication occurs. The absorption of light at wavelengths greater than about 700 nanometers is relatively low, so that a large fraction of the incident light penetrates through the p.sup.+ layer into the .pi.-type layer where the electrons generated can be swept into the multiplication region with only a small loss. At wavelengths less than about 700 nanometers a progressively larger fraction of the incident light is absorbed in the p.sup.+ layer where the electrons produced quickly recombine and are lost before they can be swept into the multiplication region. The result is a decrease in the device sensitivity of more than an order of magnitude between 700 nanometers and 400 nanometers wavelengths. To reduce the recombination losses, the thickness of the p.sup.+ layer has been reduced. However, photodiodes having a shallow p.sup.+ layer consistently exhibit high electrical noise or total failure due to susceptibility of the surface to damage from handling and/or electrical contacting.
Ohuchi et al, in U.S. Pat. No. 4,079,405 issued Mar. 14, 1978, have disclosed a semiconductor photodetector having a reduced variation in sensitivity over a wide range of wavelengths. This reduction was accomplished by reducing the thickness of the active region thereby reducing the longer wavelength sensitivity and by making the p-n junction as close as possible to the surface through which light enters the device so as to increase the sensitivity at shorter wavelengths.
It would be desirable, however, to have an avalanche photodiode in which the short wavelength sensitivity is increased while maintaining the high sensitivity at longer wavelengths.